New computer applications such as multimedia and other storage intensive applications require high areal densities and data transfer rates, but the performance of present magnetic write heads is limited by several effects which degrade performance at corresponding higher frequencies.
For example, the relatively high inductance of the write portion of current merged inductive write and magnetoresistive ("MR") heads, typically 100 to 150 nH, limits the speed at which the write current can be switched using present power supply voltages (5 to 8 volts). As a result, such heads are limited to data transfer rates of from 100 to 150 Mbits/sec.
In addition, the so-called "skin effect" becomes more significant at higher frequencies and limits typical head performance. This effect confines the magnetic flux to the outer surfaces of the magnetic layers of the device, such as the yokes and poles. This reduces magnetic efficiency, i.e., writing field per unit of write current used, and leads to premature magnetic saturation of the head. The latter effect causes problems with overwrite and non-linear transition shift ("NLTS").
As data rates increase, the time interval associated with each bit being stored on the media decreases. The write current passing through the write coil of the writing head must be switched from a positive write current value to a negative write current value well within the time interval associated with each bit. An approximate relationship between a maximum time allowed to switch this current as a function of a desired or selected data rate is shown in FIG. 1. The curve shown corresponds to allowing fifty percent (50%) of the data timing interval for write current switching, a typical value in the disk drive industry. As an example, a 500 Mbit/sec. data rate would require the current to be switched within a maximum time interval of about 1.1 nanoseconds, while 800 Mbits/sec. would require a maximum time of about 0.7 nanoseconds.
FIG. 2 summarizes a sample set of calculations for the actual write current waveform as a function of total head assembly inductance, which consists of the head inductance and any additional inductance associated with the leads. These particular calculations correspond to a circuit power supply voltage of 5 volts, which is common in the disk drive industry and a write current value of 35 mamps. Higher values of write current, as are sometimes required, would result in even longer risetimes that the values shown in FIG. 2.
FIGS. 1 and 2 clearly show that the trend towards higher data rates requires shorter and shorter write current switching times, which thereby requires lower and lower values of head inductance.
One present high performance write head is disclosed in U.S. Pat. No. 5,285,340, Ju et al., entitled "The Thin Film Magnetic Write Head with Conformable Pole Tips", issued Feb. 8, 1994, which describes a magnetic head with pole tips having vertically aligned sidewalls. As shown in FIG. 2, a magnetic head 12 includes first and second pole tips 14 and 16 of the respective first and second yoke layers 18 and 20. The magnetic head 12 is formed by first depositing the first yoke layer 18 onto a nonmagnetic substrate 22. A photoresist layer 24 is then spun atop the first yoke layer 18. An opening with vertically aligned inner sidewalls is formed within the photoresist layer 24. The first pole tip layer 14, the gap layer 26, and second pole tip layer 16 are sequentially deposited into the photoresist opening. After selective removal of the photoresist layer 24, the second yoke layer 20 is formed over the second pole tip layer 16. The magnetic head 12 of Ju et al. includes a coil layer 28 disposed on top of the photoresist layer 24. The elevated coil layer 28 necessitates the second yoke layer 20 to be formed with a curved profile. The curved second yoke layer 20 is undesirable for device fabrication and performance, as explained below.
One such fabrication problem is that of step coverage. As illustrated in FIG. 4, in a thin film structure 29, a second metallic layer 30 is deposited above a first metallic layer 32 separated by an insulating layer 34. The second metallic layer 30 must cover a large curvature profile defined by the underlying insulating layer 34. During deposition of the second metallic layer 30, the depositing material has a tendency to evenly distribute onto the depositing surface. As a consequence, the thickness of parts of layer 30 is very thin such as an area 36A, above the insulating layer 34. A similar condition exists in depositing the insulating layer 34 above the first metallic layer 32. That is, the larger the curvature profile of the deposited layer, the higher is the probability of exposing the deposited layer with areas of material weakness, such as the area 36B shown in FIG. 4. If the area with material deficiency occurs in the second metallic layer 30, there may be an open circuit. If the area of thinner material happens in the insulating layer 34, there may be an electrical short bridging the overlying and underlying layers 30 and 32. If the second metallic layer 30 is a second yoke layer, such as the layer 20 in the magnetic head 12 shown in FIG. 3, it may result in a malfunctioning head. Accordingly, in the fabrication of thin film write heads, excessive step coverage problems reduce final production yield and consequently increase manufacturing costs.
Another fabrication problem of typical heads is the difficulty of controlling the distance between the bottom of the layers 18 and 20 (FIG. 3), i.e., facing the media, and the portion of the second yoke layer 20 that curves away from the substrate 22. This distance is called the throat height and is generally desired to be formed as short as possible for the head to write with maximum magnetic field. However, if the throat height is lapped too short during processing, the gap may open and the head is nonfunctional. One means of minimizing the sensitivity of the head performance to final lapped throat height to processing is to use first and second pole tip layers 14 and 16.
Moreover, in Ju et al., the second yoke layer 20 with a high curvature profile also increases the inductance of the magnetic head 12. The reason is that the highly curved second yoke layer 20 unnecessarily lengthens the magnetic path. The longer the magnetic path, the higher is the inductance. As mentioned before, a magnetic head with yoke layers having high inductance is slow in responding to writing current and incapable of performing high rate data transfer onto media with high areal densities.
It should also be noted that disclosed in the '340 patent is a single layer coil 28. Magnetic heads are now fabricated on microscopically confined areas with limited heat dissipation capacity. Due to the inherent large inductance of the second yoke layer 20 and to increase the sensitivity of the magnetic head without injecting excessive current into the inductive coil, the number of coil windings are accordingly increased. To maintain the small physical size for a magnetic head, the coil layers are normally stacked together. The laying of additional coil layers would require additional profile curvature and exacerbate the problems explained above.
Another present high performance write head is disclosed in U.S. Pat. No. 5,438,747, Krounbi et al., entitled "Method of Making a Thin Film Merged MR Head with Aligned Pole Tips", issued Aug. 8, 1995, and is shown as magnetic head 30 in FIG. 5. The vertically aligned sidewalls of the first and second pole tips 32 and 34 are fabricated by the process of ion milling. As with the magnetic head 12 of Ju et al., the coil layer 36 of Krounbi et al. is disposed above the gap layer 38. This arrangement also results in a tall stack height covered by a highly curved second yoke layer which results in the problems explained above.
It is an object of the invention to overcome the limitations of the present thin film write heads, such as those described previously, to provide a write head that can operate at high data transfer rates. Further, it is an object to provide a head with low inductance and high magnetic efficiency. Additionally, it is an object to avoid premature magnetic saturation of the head.